Dc to DC converter method and circuitry

ABSTRACT

A DC-to-DC converter has a pulse width modulator PWM) and a hysteretic (ripple) modulator. For low current loads, the hysteretic modulator is selected; for high current loads, the PWM is selected. A mode selection switch senses the polarity of the switched output voltage at the end of each switching cycle. If the polarity changes from one cycle to the next, the mode may be instantly changed to the other mode. Counters are used to record the polarity at the end of each cycle and switching from one mode to another can be delayed by the counters to prevent changing modes based on spurious output voltage fluctuations.

CROSS-REFERNCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Serial No. 60/201,829, filed May 3, 2000.

BACKGROUND

Power supplies for computers, personal digital assistants, cellularphones and other hand held mobile electronic devices and systems haveexacting demands. Two types of converters are used to meet thesedemands. One type is a pulse width modulated (PWM) converter and theother is a hysteretic (ripple) converter. A typical single mode DC-to-DCconverter 8 with either a PWM or hysteretic controller is shown in FIG.1a. The converter 8 has a PWM controller 12 or a hysteretic controller14. The output of the controller drives the gates of output switches,typically upper and lower mosfet power transistors 16, 18. The mosfetsare connected together at a switching node 20. At high output currentlevels, the PWM controller with a synchronous rectifier providesefficient and controllable output regulation. For low output currents,the efficiency of the DC-to-DC converter operating in a fixed frequencyPWM mode gets lower because PWM switching losses become dominant. Onethe other hand, the hysteretic converter is efficient for low outputcurrents but is not efficient for high output currents.

Demands on a system can change from tens of amps to milliamps in asshort a time as a few microseconds. In order to address the variable andfrequently inconsistent current requirements, many DC-to-DC converters,especially those used in mobile systems, include both a pulse widthmodulation (PWM) converter and a hysteretic converter. Such dual modecontrollers provide a high efficiency over a wide range of load currentlevels.

A typical dual mode converter 10 is shown in FIG. 1b. The converter 10has a PWM controller 12 and a hysteretic controller 14. The output ofthe controller drives the gates of upper and lower mosfet powertransistors 16, 18. The mosfets are connected together at a switchingnode 20. The switching node 20 is connected to an inductor 22 that isconnected to a parallel network comprising an output capacitor 23 and aload as represented by resistor 24. A sense resistor 26 is connected inseries with the inductor 22. The voltage across the sense resistor iscoupled to the controller 10 to provide data on the load current. Acomparator 13 in the controller receives the voltage signal from thesense resistor 26, compares it to a reference value indicative of acritical current, and operates a switch to switch the controller betweenthe PWM modulator 12 and the hysteretic controller 14 when the sensedcurrent falls below the threshold value of the reference input to thecomparator.

At high output current levels, the PWM controller with a synchronousrectifier provides efficient and controllable output regulation. As theload current gets lower, the efficiency of the DC-to-DC converteroperating in a fixed frequency PWM mode gets lower because PWM switchinglosses become dominant. A simple hysteretic (ripple) controller improvesthe converter efficiency for light loads. The integrated circuit sensesthe load current and, when load current falls below a minimum threshold,it invokes the hysteretic (ripple) controller and disables the PWMcontroller. When the load current increases above the minimum threshold,the PWM controller resumes control. In this way high efficiency ismaintained over a wide range of load currents.

The optimal transition point for the threshold current usually lies atthe current level where the inductor current becomes “critical.”Critical current is a value of load current for which the total energystored in the inductor 22 is delivered to the load each cycle. At loadcurrents below the critical value, the inductor current must go throughzero and reverse direction at some point in the cycle. When the inductorcurrent changes direction, energy is taken from the output filtercapacitor 23 due to the bidirectional conductivity of the synchronousrectifier, lower fet 18. To maintain the output in regulation moreenergy will be delivered to the filter capacitor 23 at the nextoperating cycle. Unless the controller is switched to the hystereticcontroller 14, the converter efficiency dramatically degrades. Power andenergy are wasted. In mobile systems that rely on battery power, theoverall life of the system is likewise reduced.

To prevent the energy losses when operating at sub-critical currents,diode-like conduction is required of the lower mosfet 18. This assuresdiscontinuous inductor current operation. Operating the converter 10 inthe discontinuous conduction mode under fixed PWM mode control createsits own challenges because the small-signal loop becomes broken,closed-loop gain increases and the converter easily becomes unstable.This leads to the conclusion that hysteretic mode is preferred for safe,stable and efficient operation at sub-critical current.

In order to select the PWM or the hysteretic mode of operation, thecontroller 10 senses the load current or any current in the circuitproportional to the load current and compares the sensed load current toa reference. If the load current is higher than the reference, the PWMmode of operation is activated. Otherwise, the converter 10 operates inthe hysteretic mode. This widely used approach depends on the toleranceof the currently sensing circuitry. As the output voltages of DC-to-DCconverters for modem computer applications are getting lower and theoutput currents are getting higher and vary widely over short periods oftime, it is becoming very difficult to measure the current precisely andefficiently. This leads to increased uncertainty of the switch overpoint and, therefore, to unpredictable operation of the whole converter.

SUMMARY

The invention is a new DC-to-DC converter circuit and a method for DC toDC conversion. The circuit includes a pulse width modulator controllerand a hysteretic controller. Both controllers convert an input firstvoltage into an output second voltage during a series of repeatedswitching cycles. The circuit has a mode selection circuit for selectingone of the two controllers in accordance with the current demand of aload coupled through an inductor to the second voltage. The modeselection switch has a comparator for comparing the second voltage to areference voltage (ground) for sensing the polarity of the secondvoltage at the end of each switching cycle. The polarity of the outputvoltage at the end of the switching cycle is a measure of the state ofthe inductor. If the inductor is in continuous operation and the loadcurrent is above the critical current, then the polarity of the switchnode will be positive. If the inductor is in discontinuous operation,then the polarity of the switch node will be negative.

One or more counters are coupled to the comparator and to the modeselection switch. The counters record the polarity of the second voltageat the end of each switching cycle and keep that data for a given numberof cycles. If the polarity of the switch node does not change, then thecontroller remains in whichever mode (PWM or hysteretic) that it hasbeen operating in. By using counters, the invention avoids prematureswitching for a single change in polarity. Such changes may occur forspurious reasons that are not related to enduring load conditions. Assuch, the counters maintain the mode selection switch in its currentmode so long as the polarity of the second voltage at the end of eachcycle does not change. However, when the polarity changes and thechanges endure for more than n switching cycles, then the countersoperate the mode selection switch to change the mode of operation to theother mode. If the converter was operating in the PWM mode, then it isswitched to the hysteretic mode and vice versa.

The DC-to-DC converter circuit operates to switch the mode selectionswitch to the hysteretic mode controller when the polarity of the secondvoltage for n number of cycles is positive and to switch the modeselection switch to the pulse width modulator controller when thepolarity of the second voltage for n number of cycles is negative. Thereare separate counters for counting the positive and negative cycles. Thenumber n may be the same or different for both counters.

DESCRIPTION OF THE DRAWING

FIG. 1a is a schematic of a single mode DC-to-DC converter.

FIG. 1b is a schematic of a dual mod DC-to-DC converter.

FIGS. 2a-2 c are graphs of the converter performance during continuousconduction.

FIGS. 2d-2 f are graphs of the converter performance duringdiscontinuous conduction.

FIGS. 3 are graphs of the invention operation during PWM and hystereticoperations.

FIG. 4 is a schematic of a mode-controlled DC-to-DC controller.

FIG. 5 is a detailed schematic of a mode-controlled DC-to-DC controllerthat includes the invention.

DETAILED DESCRIPTION OF THE INVENTION

The new method and circuit to control the operation mode of a DC to DCconverter are based on detection of the difference in the voltagewaveform on the switching node of the converter. The polarity of theswitch node voltage at the end of a switch cycle depends upon whetherthe filter inductor current is continuous (and therefore above thecritical current) or discontinuous (and below the critical current).FIGS. 2a-2 c show conditions where there is continuous inductor current.The switching cycle ends with a negative voltage on the switching node.FIGS. 2d-2 f show conditions where the inductor current is discontinuousand the voltage at the end of the switching cycle is a positive voltage.In a simple buck converter, with only a diode for low-side currentconduction, the switching node voltage at the end of the cycle will ringto a relatively large voltage greater than the output voltage of theregulator, In a synchronous rectifier buck converter, the currentreversal forces the switching node to a small positive voltagedetermined by the resistance of the synchronous switch.

Turning to FIG. 4, the dual mode converter 100 of the invention has amode control switch 50 that monitors the polarity of the voltage at theswitching node 20 between the upper and lower mosfets 16, 18. Thepolarity of the output voltage could be monitored at a number of otherlocations. For example, one skilled in the art could insert a small (afew milliohms) resistor in series between the lower mosfet 18 andground. FIG. 3, top, shows the DC output voltage, during a PWM mode anda subsequent hysteretic mode. When the converter operates in PWM modeand load current decreases, the inductor current I_(IND) gradually fallsand the polarity signal (phase comparator) has wider pulses. A counterin the mode switch 50 counts the number of pulses of the new polarityand after n pulses (in this case n=8) in a row, the mode switches fromPWM to hysteretic. A corresponding switch from hysteretic to PWM is madewhen the inductor current I_(IND) increases and the polarity shiftsagain. An embodiment of the counters and logic circuitry are shown inFIG. 5 and discussed below.

The invention uses a finite storage interval of n switching cycles. Ifthe sign of the switching node voltage at the end of each of n cyclesremains unchanged, its sign is used to determine the subsequent mode ofoperation. A positive voltage on the switching node corresponds with thehysteretic operating mode; a negative voltage corresponds to the pulsewidth modulation mode. If at least once during these n switching cycles,the sign of the switching node changes, the counter is reset and theoperating mode of the converter remains unchanged. The monitoringprocess constantly repeats itself while the converter is operating.

FIG. 5 shows a detailed schematic of a possible embodiment of the modecontrol switch with counters for and polarity sensors for distinguishingcontinuous from discontinuous currents in the inductor. The mode switch50 includes a comparator 52 that compares the switching node to ground.The output of comparator 52 is clocked to the input of D-flip-flop 54 bythe PWM signal. Two counters 56, 57 count to eight cycles, one countingpositive switch node voltage, the other, negative. The counters 56, 57are reset when the clocked switch node polarity (Q output of 54)changes. In the preferred embodiment each counter uses a count of eightto invoke the appropriate operating mode. However, the number of countsis arbitrary. The mode switch circuit 50 also provides a means tooverride the HYSTERETIC-to-PWM delay or counter in order to permit fastresponse to load current transients. The embodiment of this means in themode switch control 50 uses a comparator 58 for monitoring the outputfeedback voltage and causing an immediate resetting of the MODE latchout of the HYSTERETIC mode when the feedback voltage is below thereference by an amount set to be greater than the normal ripple voltageexpected. In other words, the delay of eight cycles in going fromHYSTERETIC to PWM mode can be entirely eliminated via this means. A PWMreset comparator 58 compares the output voltage on the load VFB to areference voltage. If VFB is greater than the reference, the circuit 50immediately switches to the PWM mode. The reference is set to detectlarge output voltages. If the triggering voltage is spurious, thecircuit 50 will reset to the HYSTERETIC mode after n cycles.

In operation, the invention provides a method to control the operationof a DC-to-DC converter 10 based on monitoring the waveform on thejunction node 20 of the mosfets 16, 18 and the filter inductor. The modeswitch controls the operation mode of a DC-to-DC converter based onmonitoring the polarity of the voltage waveform on the junction node 20of the mosfet switches 16,18 and the filter inductor 22. The voltage onthis switching node 20 at the end of a switching cycle is detected and asignal indicating the sign of this voltage is stored for a finiteinterval of time, which is longer than one switching cycle anddetermined by practical considerations. If during this interval of time,there are several switching cycles all of the same sign of voltage atthe switching node at the end of the switching cycle, a decision is madethat the operating mode will be in correspondence with the stored signof the voltage on the switching node. Positive voltage corresponds witha hysteretic operating mode. The negative sign corresponds with the PWMoperating mode.

If at least once during the measuring time interval, the sign of theswitching node changes, the operating mode of the converter remainsunchanged. The monitoring process constantly repeats itself while theconverter is operating.

Having thus described the preferred embodiment of the invention, thoseskilled in the art will understand that further changes, additions,deletions and modifications may be made to this embodiment withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

What we claim is:
 1. A DC-to-DC converter circuit comprising: a pulsewidth modulator controller and a hysteretic controller, each controllerfor converting a first voltage into a second voltage during a series ofrepeated switching cycles; a mode selection circuit for selecting one ofthe two controllers in accordance with the current demand of a loadcoupled to the second voltage; a comparator for comparing the secondvoltage to a reference voltage and generating a polarity signalrepresentative of the second voltage at the end of each switching cycle;and one or more counters coupled to the comparator and to the modeselection switch for recording the polarity of the second voltage at theend of each switching cycle, for maintaining the mode selection switchin its current mode so long as the polarity of the second voltage at theend of the each cycle is the same as the last n number of cycles and foroperating the mode selection switch to select the other controller whenthe polarity of the second voltage at the end of the cycle changes fromthe prior cycle and the change in polarity persists for n number ofcycles.
 2. The DC-to-DC converter circuit of claim 1 wherein thecounters operate to switch the mode selection switch to the hystereticmode controller when the polarity of the second voltage for n number ofcycles is positive.
 3. The DC-to-DC converter circuit of claim 1 whereinthe counters operate to switch the mode selection switch to the pulsewidth modulator controller when the polarity of the second voltage for nnumber of cycles is negative.
 4. A DC-to-DC converter comprising: meansfor pulse width modulating and ripple modulating a first voltage toconvert the first voltage into a second voltage during a series ofrepeated switching cycles; means for selecting one of the two modulatingmeans in accordance with the current demand of a load coupled to thesecond voltage; means for sensing the polarity of the second voltage atthe end of the switching cycles; and means for switching from onemodulating means to the other modulating means when the polarity of thesecond voltage changes at the end of the switching cycle.
 5. Theconverter of claim 4 wherein the means for sensing the polarity of thesecond voltage comprises means or comparing the second voltage to apolarity reference voltage; means for generating a polarity signalrepresentative of the polarity second voltage at the end of eachswitching cycle; means for recording the polarity of the second voltageat the end of each switching cycle for n number of cycles; means formaintaining the current modulating means so long as the polarity of thesecond voltage at the end of the each cycle is the same as the last nnumber of cycles; and means for selecting the other modulation meanswhen the polarity of the second voltage at the end of the cycle changesfrom the prior cycle and the change in polarity persists for n number ofcycles.
 6. The converter of claim 4 wherein the means for selectingselects the ripple modulation means when the polarity of the secondvoltage for n number of cycles is positive.
 7. The converter of claim 4wherein the means for selecting selects the pulse width modulation meanswhen the polarity of the second voltage for n number of cycles isnegative.
 8. A method for converting one DC voltage into another DCvoltage comprising: pulse width modulator mode and a hystereticmodulator mode for generating a second voltage during a series ofrepeated switching cycles; selecting one of the two modes in accordancewith the current demand of a load coupled to the second voltage; sensingthe polarity of the second voltage at the end of the switching cycles;and maintaining the current mode so long as the polarity of the secondvoltage at the end of the each cycle is the same as the last n number ofcycles; and selecting the other mode when the polarity of the secondvoltage at the end of the cycle changes from the prior cycle and thechange in polarity persists for n number of cycles.
 9. The method ofclaim 8 wherein the hysteretic mode is selected when the polarity of thesecond voltage for n number of cycles is positive.
 10. The method ofclaim 9 wherein the step of sensing the polarity of the second voltagecomprises: comparing the second voltage to a polarity reference voltage;generating a polarity signal representative of the polarity secondvoltage at the end of each switching cycle; and recording the polarityof the second voltage at the end of each switching cycle.
 11. The methodof claim 8 wherein the pulse width modulation mode is selected when thepolarity of the second voltage for n number of cycles is negative.